Transport of Oxygen & Carbon Dioxide

Transport of Oxygen & Carbon Dioxide

• Oxygen Transport:

  • Occurs in three stages:
    • Diffusion of O₂ from alveoli into pulmonary blood.
    • Transport of blood through arteries to tissue capillaries.
    • Diffusion of O₂ from the capillaries to tissue cells.

• Oxygen Carriage:

  • Majority of oxygen is carried in red blood cells on iron/haem molecules in haemoglobin.
  • 1.5% is dissolved in the plasma.
  • Oxyhaemoglobin = Hb saturated with O₂
  • Deoxyhaemoglobin = Hb without O₂

• Haemoglobin Structure:

  • Each haemoglobin protein consists of 4 haem molecules.
  • Each haem molecule combines with 2 oxygen molecules (Hb₄O₈).
  • Each red blood cell carries ~280 million Hb proteins.

• Haemoglobin Function:

  • Uploads O₂ when plentiful.
  • Easily transports O₂ without offloading unnecessarily.
  • Offloads O₂ when needed, adjusting the amount to meet demand.

• Oxygen Saturation:

  • Measured as SaO₂ (arterial O₂ saturation) by a blood gas machine or SpO₂ (peripheral O₂ saturation) by a pulse oximeter.
  • Normal value is 95-99%.

• Gaseous Pressure in Alveoli & Blood:

  • Alveolar air: O₂ = ~104mmHg/14kPa, CO₂ = ~40mmHg/5.3kPa, N₂=~569mmHg/78kPa.
  • Venous blood: O₂ = ~40mmHg/5.3kPa, CO₂ = ~45mmHg/6kPa, N₂ = ~569mmHg/78kPa.
  • Arterial blood: O₂ = ~100mmHg/13.3kPa, CO₂ = ~40mmHg/5.3kPa, N₂ = ~569mmHg/78kPa

• Affinity:

  • Affinity is binding ability.
  • As O₂ binds to Hb, affinity for O₂ increases.
  • As O₂ is released, Hb affinity for O₂ decreases.
  • High affinity helps pick up O₂ in the lungs.
  • Lower affinity helps release O₂ to tissues.

• Partial Pressure:

  • Partial pressure is the pressure a gas would exert if it occupied the space alone.
  • Total pressure of a gas mixture is the sum of the partial pressures of individual gases in the mixture.

• Oxygen Dissociation Curve:

  • Shows the percentage of haemoglobin combined with oxygen at different oxygen pressures.
  • Lower partial pressure of oxygen (pO₂), lower affinity of Hb for O₂ and O₂ is released to tissues.

Other Considerations

• Myoglobin:

  • Oxygen storage protein in slow-twitch skeletal and cardiac muscle.
  • Stores 1/4 the amount of oxygen as haemoglobin.
  • Important in initial phase of exercise to supply mitochondria with oxygen.
  • Important in oxygen supply to heart muscle during systole and coronary occlusions.

• Fetal Haemoglobin:

  • Higher affinity for oxygen than adult haemoglobin.
  • Facilitates oxygen transfer from maternal blood to fetal blood in the placenta.

Carbon Dioxide Transport

• Carbon Dioxide Transport:

  • 70% transported as bicarbonate ions (HCO₃⁻).
  • 10% dissolved as gas molecules directly in the blood.
  • 20% as carbaminohaemoglobin, bound to Hb.

• CO₂ Equation:

  • CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻
  • Carbon dioxide and water react reversibly to form carbonic acid, which dissociates into hydrogen ions and bicarbonate ions.

• Haldane Effect:

  • The lower the PO₂ (and Hb saturation with O₂), the more CO₂ can be carried by the Hb.
  • Deoxyhaemoglobin has a higher affinity for CO₂ than oxyhaemoglobin.

• CO₂ and O₂ exchange in the lungs and tissues:

  • At the lungs, CO₂ diffuses from blood into alveolar air.
  • At the tissues, CO₂ diffuses from tissue cells into blood.

Partial Pressures of O₂ and CO₂ in Blood and Alveoli

• Arterial Blood Gases:

  • pH: 7.35 - 7.45
  • PaCO₂: 4.6 - 6.0 kPa (35 – 45 mmHg)
  • PaO₂: 11 - 14 kPa (80 – 100 mmHg)
  • HCO₃⁻: 22 – 26 mmol/L
  • SaO₂: 95–98%

• Venous Blood Gases:

  • pH: 7.34 - 7.37
  • P_vCO₂: 5.8 – 6.1 kPa (44 – 46 mmHg)
  • P_vO₂: 5.0 – 5.5 kPa (38 – 42 mmHg)
  • HCO₃⁻: 24 – 30 mmol/L
  • SvO₂: ~75%

• Ventilation Control:

  • Respiratory centres: Chemoreceptors (peripheral and central), Baroreceptors in the aortic arch, Stretch receptors that prevent overinflation.
  • Reaction to irritants: Irritants in the respiratory tract trigger reactions like coughing, sneezing, and laryngospasm.
  • Voluntary control: Anticipation of exercise, and partial control.

• Control of respiratory rate and depth: Respiratory centers in the brainstem set the rate and depth of breathing. Centres receive input from sensory neurons to alter the ventilatory pattern.

• Chemoreceptors: Measure pH and levels of gases in blood. Central Receptors: Medullary receptors sensitive to H+ and rise in CO2 increases rate & depth of breath. Peripheral Receptors in the aortic and carotid bodies are sensitive to lower P02, increased PCO2 & increased H+.

• Baroreceptors: measure blood pressure. When blood pressure falls, ventilation increases and vice versa.